Current transformer



March 1963 J. L. SETTLES ETAL 3,082,389

CURRENT TRANSFORMER Filed Jan. 25, 1960 INVENTORS James L. Settles 8Edmond E. Conn BY; 'ATTORNEY 94 {M}. Mi-

United States Patent 3,082,389 CURRENT TRANSFORMER James L. Settles,Sharon, Pa., and Edmond E. Conner,

Brookfield, Ghio, assignors to Westinghouse Electric Corporation, EastPittsburgh, Pa, a corporation of Pennsylvania Filed Jan. 25, 1969, Ser.No. 4,288 4 Claims. (Cl. 336-174) This invention relates to electricalinduction apparatus and, more particularly, to current transformers.

In conventional current transformers of the types which are known as thethrough-type and the bar-type, the

effective primary win ing comprises a current-carrying electricalconductor, which may take the form of a bartype electrical conductorwhich is provided as part of the current transformer assembly, and asecondary winding which is toroidal in configuration and which isdisposed in inductive relationship with said primary winding. Sufficentelectrical insulation must be provided between the effective primarywinding and the secondary winding of a current transformer of the typedescribed, the amount and the type of electrical insulation dependingupon the voltage of the electrical circuit with which the currenttransformer is to be employed. It is, therefore, desirable to provideand improved current transformer of the type described which morereadily lends itself to convenient assembly for application at wherevercircuit voltage is involved in a predetermined operating range ofoperating voltages.

It is an object of this invention to provide a new and improved currenttransformer.

Another object of this invention is to provide a new and improvedcurrent transformer which more readily lends itself to assembly forapplication at a predetermined voltage within a predetermined range ofvoltages.

A further object of this invention is to provide a new and improvedcurrent transformer in which the major insulation is formed fromresilient, elastomeric material.

Other objects of the invention will, in part, be obvious and will, inpart, appear hereinafter.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompaning drawing in which:

FIGURE l'is a top plan view of a current transformer embodying theteachings of the invention;

FIG. 2 is a side elevational view of the current transformer shown inFIG. 1; and

FIG. 3 is an end elevational view, partly in section, of the currenttransformer shown in FIG. 1.

Referring now to the drawing and FIGS. 2 and 3 in particular, thisinvention is illustrated by reference to a current transformer showngenerally at 10. As illustrated in this embodiment, the currenttransformer comprises winding 72 from a resilient, elastomeric,electrically insulating, thermosettable composition or material 82, suchas a polyester resin of the type disclosed in US. Patent 2,871 ,420 byH. F. Minter et al. which issued January 27, 1959, and which is assignedto the same assignee as the present application, and in the copendingapplications Serial No. 647,748, filed March 22, 1957, and Serial No.745,847, filed July 1, 1958, by H. F. Minter and also assigned to thesame assignee as the present'application. The casing 30 includes anaxial opening or aperture/therethrough, as indicated at 46, which isadapted to receive the associated insulating bushing 20, as will beexplained hereinafter.

In order to support the core 74 and the toroidal secondary winding 72during the forming or molding of the casing 30, and for other purposeswhichwill be described hereinafter, a generally ring-shaped bracketmember 116 the magnetic core 74 which may be of the ring or v toroidaltype, the secondary winding 72 which is also toroidal in shape and whichis inductively disposed on the magnetic core 74, the insulating casing30 which is formed or molded around said core and secondary winding, andthe insulating bushing 20 which passes through an opening in said casingand through which an associated primary winding (not shown), such as acurrent carrying conductor, passes. The current transformer 10 isillustrated as being of the type known in the art as the through-type.

After the torodial winding 72 is assembled ,on the associated magneticcore 74 as best shown in FIG. 3, the insulating casing or housing 30which is generally cylindrical in shape is preferably formed by asuitable method, such as molding, about the core 74 and the secondary isprovided which extends nearly around the outer periphery of the toroidalwinding 72. and which extends axially along the longitudinal dimensionof said core and secondary winding. To assist the bracket member 116 insupporting the core 74- and the secondary winding 72, the supportingmembers or feet 112 are disposed at each side of the casing 30 andwelded or otherwise secured to the underside of the bracket member 116,as best shown in FIG. 3. The casing 30 is also provided with a pluralityof smaller openings or apertures 53 which extend axially through saidcasing to receive the bolt means 52 for reasons which will be discusedhereinafter. Each of the openings 53 may be provided with a suitableliner or tube member 114 which is preferably formed from a suitablecorrosion-resistant material, such as brass, and which may be welded orotherwise secured to the bracket member 116 prior to the molding orforming of the casing 30. Each of the supporting members or feet 112 maybe provided with suitable means, such as the projection weld nuts 118,which are adapted to receive suitable fastening means such as bolts (notshown) for securing the associated mounting bracket or base member 42,which may be of any conventional type, to the bottom of the casing 30,as best shown in FIGS. 2 and 3. The casing 30 may also include aterminal portion formed integrally therewith or a separate terminalcover64 into which the terminals of the secondary winding 72 are brought andwhich is secured or fastened to the casing 30 by suitable means, such asthe stud or bolt means 62.

In order to make up the necessary electrical insulation required toapply the current transformer 10 at a particular voltage, the insulatingbushing 20 is provided to add to or supplement the electrical insulationof the casing 30. The bushing 20, which is generally cylindrical inshape, is illustratedjas being of the hollow cylindrical or tubular typehaving an axial opening or aperture therethrough, as indicated at 75.The bushing 20 is preferably formed by a suitable method, such asmolding, from a resilient, elastomeric, electrically insulatingcomposition of material having anti-tracking properties, such as butylrubber with an aluminum trihydrate filler.

in particular, the bushing 21) comprises a center or rent transformermay be applied, said end portions each including a plurality of ridgeportions or petticoats 43 which extend around the outer periphery ofsaid bushing and which, in this instance, are uniformly spaced apartalong the axis of the bushing 20. The ridges 43 of the bushing 29 arespaced apart by a distance equal to the spaces between said ridges, asindicated at 45 in FIG. 1. The outer diameter or peripheral dimension ofthe intermediate portion 20C of the bushing 20 is arranged to be lessthan the diameter or peripheral dimension of the opening or aperture 46in the casing 30 so that the bushing 20 may readily pass through saidopening when said bush ing is assembled with the casing 30. It is to benoted that the casing 30 may be readily assembled with a variety ofbushings similar to the bushing 20 and having an outer diameter orperipheral dimension less than the diameter or peripheral dimension ofthe opening 46 in the casing 30 and having any suitable axial dimensionrequired to provide the insulation supplemental to that of the casing 30for application of the current transformer at a particular voltage. Thebushing is preferably molded or formed about a suitable'supportingmember, such as the 'hollow cylindrical or tubular member 98, which isprefe ably formed from a non-magnetic material, such as brass, havingthe necessary mechanical strength to support the end portions 20A and20B of the bushing 2t) when said bushing is assembled with the casing30.

As mentioned previously, the bushing 20 is adapted to receive theeffective primary winding of the current transformer 10, which may beany conventional type of current-carrying conductor of a size suitableto pass through the tubular member 98 of the bushing 20. It is to beunderstood, however, that the current transformer 10 may be providedwith a bushing similar to the bushing 20 having a generally cylindricalshape in which the resilient, elastomeric, electrically insulatingmaterial is molded or formed about a suitable current carrying conductorsuch as that of the bar-type to form a current transformer similar tothe current transformer 10, but of the bar-type rather than of thethrough-type as illustrated. The current-carrying conductor in thelatter construction would normally be adapted for coupling or connectionto the current-carrying conductors of an associated electrical circuit.

In order to hold the bushing 20 in assembled relationship with thecasing 30 of the current transformer 10, the current transformer 10 alsoincludes the clamping means 22 and 24 which are disposed at the oppositeends of the casing 30 adjacent to the opening 46 in said casing andwhich are of the split ring type. The clamping means 24 includes the twogenerally C-shaped clamping members 24A and 248 which are arranged tosubstantially surround the outer periphery of the bushing 20 adjacent tothe opening 46 of the casing 30 and which are secured to the casing 30by the bolt means 52. Similarly, the clamping means 22 at the oppositeend of the casing 30 includes the two generally C-shaped clampingmembers 22A and 22B which are arranged to substantially surround theouter periphery of the bushing 20 adjacent to the opening 46 of saidcasing and which are similarly secured to the casing 30 by the boltmeans 52 which extend through the openings or apertures 53 in saidcasing, as best shown in FIGS. 1 and 2. The clamping means 22 and 24 arearranged or adapted to apply axial clamping forces to the intermediateportion 20C of the bushing 20 in opposite directions to hold saidbushing in assembled relationship with the casing 30 as just mentioned.In order to prevent rotation of the bushing 20 with respect to thecasing 30 after said bushing is assembled with said casing, theintermediate portion 200 of the bushing 20' is provided with one or moreprojecting portions or lugs 96 at each end thereof which are engageableby the ends of the associated clamping members 22A and 22B and 24A and2413, respectively, at the opposite ends of the casing 30, as best shownin FIG. 3, if the bushing 20 should tend to rotate in place afterassembly with the casing 30, It is to be noted that the inner diameteror peripheral dimension of the closed loop defined or formed by theclamping means 22 and 24, respectively, at the opposite ends of thecasing 36 is conveniently arranged to be greater than the outer diameteror peripheral dimension of the bushing 20, as indicated in the spaces45, but less than the outer diameter or peripheral dimension of thebushing 20 as taken at the top of the ridges 43.

It is important to note that the insulating casing 30 provides aninsulation level which permits the application of the currenttransformer 10 at a first predetermined voltage even without theadditional insulation provided by the insulating bushing 20. Theinsulating bushing 20 when assembled with the casing 30 in the mannerpreviously described, then provides additional insulation whichcooperates with the insulation of the casing 30 to permit theapplication of the current transformer 10 at voltages higher than thefirst predetermined voltage just mentioned and which varies with theaxial dimension of the insulating bushing 29. The manufacture orassembly of the current transformer 10 may therefore be convenientlyaccomplished in two steps. In the first step, the secondary winding 72is assembled on the associated magnetic core 74 and then an insulatingcasing 30 is formed or molded about said core and winding as previouslydescribed. The second main step in the assembly of the currenttransformer 10 is to select a suitable insulating bushing similar to thebushing 20 in order to provide the additional insulation required forapplication of the transformer 10 at a particular voltage in an overallrange of voltages. The insulating bushing 20 of a suitable axialdimension is then assembled with the casing 30 in the manner justdescribed. It is important to note that if the current transformer 10 isto be applied at a different voltage, only the insulating bushingsimilar to the bushing 20 need be changed by removing the clamping means22 and 24 and then assembling the casing 30 with a new insulatingbushing as required for application at a particular voltage.

Although the clamping means 22 and 24 have been described in detail, itis to be understood that other suitable means may be employed to hold orsecure the bushing 20 and the casing 30 in proper assembledrelationship.

The apparatus embodying the teachings of this invention has severaladvantages. For example, a current transformer as disclosed more readilylends itself to convenient assembly for application at differentvoltages in a predetermined range of voltages. In addition, a cur renttransformer as disclosed readily lends itself to application at adifferent voltage by the relatively simple operation of changing theinsulating bushing which forms part of the overall assembled currenttransformer as described. Finally, a current transformer as disclosedpermits a reduction in the number of subassemblies or components whichmust be available to provide a Wide variety of current transformerratings.

Since numerous changes may be made in the abovedescribed apparatus anddifferent embodiments of the invention may be made without departingfrom the spirit and scope thereof, it is intended that all the mattercon tained in the foregoing description or shown in the accompanyingdrawing shall be interpreted as illustrative and not in a limitingsense.

We claim as our invention:

1. A current transformer comprising a closed loop magnetic core, asecondary winding inductively disposed on said core, a casing having anaperture therethrough molded around said core and winding from aresilient, elastomeric, electrically insulating, thermosetting material,a cylindrical bushing having an intermediate portion disposed to passthrough said aperture and first and second end portions extendinglongitudinally in each direction outwardly from said aperture, saidbushing being molded from a resilient, elastomeric electricallyinsulating material, and clamping means secured to said casing at eachend thereof for applying longitudinal clamping forces to said bushing tohold said casing and said bushing in assembled relationship, saidclamping means comprising split clamping rings disposed to substantiallysurround said bushing at each end of said casing adjacent to saidopening and to bear longitudinally against the intermediate portion ofsaid bushing.

2. A through-type current transformer comprising a toroidal magneticcore having a secondary winding disposed thereon and an axial openingtherethrough, a casing molded around said core and Winding from aresilient, elastomeric, electrically insulating, thermosettable resin, ahollow cylindrical bushing having an intermediate portion disposedsubstantially inside said opening and end portions extending outwardlyin each direction from said opening, said bushing being formed from aresilient, elastomeric, electrically insulating material, and a pair ofgenerally C-shaped clamping members secured to said casing adjacent toeach end of said opening to substantially surround said bushing, saidclamping members being arranged to apply axialclarnping forces to theintermediate portion of said bushing to maintain said casing and bushingin assembled relationship.

3. A through-type current transformer comprising a toroidal magneticcore having a secondary Winding disposed thereon and an axial openingtherethrough, a casing molded around said core'and Winding from aresilient, elastomeric, electrically insulating, thermosettable resin, ahollow cylindrical bushing having an intermediate portion disposedsubstantially inside said opening and end portions extending in eachdirection outwardly from said opening, said bushing being formed from aresilient, elastomeric, electrically insulating material, and a pair ofgenerally C-shaped clamping members secured to said casing adjacent toeach end of said opening to substantially surround said bushing, saidclamping members being arranged to apply axial clamping forces to theintermediate portion of said bushing to maintain said casing and bushingin assembled relationship, said intermediate portion including at leastone axial projection at each end of said casing engageable by saidclamping members to prevent rotation of said bushing with respect tosaid casing.

4. A transformer comprising a toroidal magnetic core having a Windingdisposed thereon and an axial opening therethrough, a casing moldedaround said core and winding from an elastomeric, electricallyinsulating, thermosetting material, a cylindrical bushing having anintermediate portion disposed substantially inside said opening and endportions extending outwardly from said opening in each axial directionand each including a plurality of alternate ridge portions and spaces onthe outer periphery thereof, said bushing being formed from anelectrically insulating, elastorneric material, and a pair of generallyC-shaped clamping members secured to each end of said casing tosubstantially surround said bushing, the inner peripheral dimension ofthe opening defined by each pair of said clamping members being greaterthan the outer peripheral dimension of said bushing in said spaces butless than the outer peripheral dimension .of said bushing at said ridgeportions, said clamping References Cited in the tile of this patentUNITED STATES PATENTS 2,929,037 Breth et al. Mar. 15, 1960

1. A CURRENT TRANSFORMER COMPRISING A CLOSED LOOP MAGNETIC CORE, ASECONDARY WINDING INDUCTIVELY DISPOSED ON SAID CORE, A CASING HAVING ANAPERTURE THERETHROUGH MOLDED AROUND SAID CORE AND WINDING FROM ARESILIENT, ELASTOMERIC, ELECTRICALLY INSULATING, THERMOSETTING MATERIAL,A CYLINDRICAL BUSHING HAVING AN INTERMEDIATE PORTION DISPOSED TO PASSTHROUGH SAID APERTURE AND FIRST AND SECOND END PORTIONS EXTENDINGLONGITUDINALLY IN EACH DIRECTION OUTWARDLY FROM SAID APERTURE, SAIDBUSHING BEING MOLDED FROM A RESILIENT, ELASTOMERIC ELECTRICALLYINSULATING MATERIAL, AND CLAMPING MEANS SECURED TO SAID CASING AT EACHEND THEREOF FOR APPLYING LONGITUDINAL CLAMPING FORCES TO SAID BUSHING TOHOLD SAID CASING AND SAID BUSHING IN ASSEMBLED RELATIONSHIP, SAIDCLAMPING MEANS COMPRISING SPLIT CLAMPING RINGS DISPOSED TO SUBSTANTIALLYSURROUND SAID BUSHING AT EACH END OF SAID CASING ADJACENT TO SAIDOPENING AND TO BEAR LONGITUDINALLY AGAINST THE INTERMEDIATE PORTION OFSAID BUSHING.